A plasmon is a vibrational quantum of free electrons in metal or like substance. It is possible to vibrate free electrons in metal as a group, and to excite plasmon by irradiating light onto a nano-scale structure of metal (metal nano-structure).
It is possible to obtain a locally enhanced electromagnetic field by exciting plasmon. In view of the above, various applications utilizing plasmon energy have been expected.
As one of the application examples, there is proposed a technology, in which local plasmon is excited by incidence of light onto a metal nano-structure, the optical electric field near the metal nano-structure is locally enhanced, and information is recorded in a very small area in the order of nanometers exceeding the diffraction limit, with use of the enhanced optical electric field, for the purpose of implementing super high density information recording or reproduction by light (see patent literature 1, for instance).
FIG. 20 is a diagram showing a configuration of a conventional optical pickup for use in recording or reproducing information to or from an information medium with use of plasmon, which is disclosed in patent literature 1.
Referring to FIG. 20, laser light 101 emitted from a laser light source 100 is converged through a lens 102, and is collected on a metal nano-structure 104 formed on a surface of a substrate 103 made of a material capable of transmitting the laser light 101, from the back side of the substrate 103.
The metal nano-structure 104 is embedded in the substrate 103 so as not to obstruct the movement of the substrate 103 relative to a recording medium 105. In patent literature 1, a columnar-shaped hole of about 50 nm in diameter and about 100 nm in depth is formed in the substrate 103, and the metal nano-structure 104 made of gold is embedded in the hole.
The lens 102 is finely movable upward and downward, and leftward and rightward with respect to the substrate 103 by an unillustrated mechanism. The position of the lens 102 is adjusted and fixed so that the middle part of the focal point of the lens 102 is aligned with the metal nano-structure 104.
When the laser light 101 is entered to the lens 102, local plasmon is excited by the metal nano-structure 104, and the optical electric field near the metal nano-structure 104 is enhanced.
The metal nano-structure 104 disclosed in patent literature 1 has a columnar shape, and the bottom surface of the metal nano-structure 104 lies in a flat plane of the substrate 103 facing a surface of the recording medium 105. Accordingly, the spread of the enhanced optical electric field generally corresponds to the diameter of the bottom surface of the metal nano-structure 104, namely, about 50 nm.
Loading an optical pickup in a recording or reproducing device having a function of controlling the distance between the metal nano-structure 104 and the recording medium 105, and bringing the metal nano-structure 104 in proximity to the recording medium 105 by a predetermined distance makes it possible to record information with a spot diameter generally corresponding to the spread of the enhanced optical electric field.
In reproducing recorded information, incident light of an intensity weaker than the intensity of incident light used at the time of recording is used, and a signal representing light reflected from or transmitted through the metal nano-structure 104 is detected.
The threshold value for signal detection is set in a range between a signal intensity representing light from a plasmon-enhanced optical electric field, and a signal intensity representing light collected through the lens 102 at a position other than the optical electric field so that a signal other than the signal representing the light from the plasmon-enhanced optical electric field is not detected. According to the above configuration, it is possible to reproduce the information written in a very small area equal to or smaller than the diffraction limit.
In the conventional configuration shown in FIG. 20, the plasmon-enhanced optical electric field drastically decreases exponentially, as the optical electric field is distanced away from the metal nano-structure 104. In view of the above, it is necessary to minimize the distance between the optical pickup and the recording medium 105 i.e. the operating distance in the order of several nanometers, for instance, in order to allow the optical electric field to reach the recording medium 105 for information recording or reproduction.
Further, it is necessary to keep the operating distance to a constant value with extremely high precision in the order of sub-nanometers. In the case where it is insufficient to keep the operating distance to a constant value, as the operating distance varies, the intensity of a laser beam to be irradiated onto a signal plane of the recording medium 105 may greatly vary. This may deteriorate the recording precision or the reproduction precision.
As a configuration for reducing the operating distance, there is proposed a configuration substantially equivalent to a magnetic head in a conventional hard disk device, for instance. Specifically, there is proposed an idea of lifting a light collecting optical system for generating plasmon light, which is attached to a tip end of a swing arm, above a recording medium, using an airstream generated by rotating the recording medium.
According to the above configuration, however, the strength of airstream depends on a linear velocity of a recording medium. Accordingly, a slight fluctuation of the linear velocity of a recording medium may vary the operating distance in the order of nanometers or in the order of sub-nanometers. This may resultantly vary the recording light amount or the reproduction light amount.
Further, the operating distance may vary due to a temperature change or due to an external disturbance. In view of the above, it is necessary to devise a novel approach for detecting an operating distance for feedback control in order to keep the operating distance constantly with ultra-high precision.